bug#22000: Patch addressing the menu-bar frame-resize interaction

[Vivek Dasmohapatra]

GTK 3.16 and later:

The menu bar is always in a scrollwed window . In the default mode,
the menu bar is truncated when it tries to grow wider than the frame.
CSS is used to strip away the excess space this introduces.

In 'always or 'automatic mode, the CSS is relaxed slightly to work
around a GTK focus glitch, but otherwise the widget setup is identical.
The menubar will have a scrollbar either always, or when it tries to
grow too wide.

Before GTK 3.16:

When in 'always or 'automatic mode, the menu bar will be in a scrolled
window. The extra space cannot be properly ameliorated with CSS
styling as this does not seem to work well. In the default mode,
the scrolled window is not present - the menu bar is dynamically
re-parented between the scrolled window (which is created on demand)
and the emacs pane (vbox widget) when the menu bar scrolling mode
is changed.

At these GTK versions truncation does not work, so the menu bar
frame size jitter big persists in the default mode.

[ Tested on gtk 3.22.11 and 3.4.2 ]

emacs-frame-size-jitter-rc0.tar.xz
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